Investors in JP Morgan Chase had a whale of a time on Friday as the bank's shares surged. But its earnings showed yet again why investing in big US banks can be so frustrating.

The bank's nearly $5bn, second-quarter net profit blew past expectations. And it seemed to validate chief James Dimon's assertion that JP Morgan would be "solidly profitable" despite the trading debacle that led to $5.8bn in losses in the first half, of which $4.4bn hit in the second quarter.

At the same time, Dimon said the bank has put behind it most of the issues related to its Chief Investment Office. JP Morgan said maximum future losses from the unit's botched trades could range from $800m to $1.6bn, but that the upper end represented a worst-case scenario and that the remaining positions could even end up making money from here.

Though markets cheered, JP Morgan's shares are still down more than 15% since the bank announced the CIO problem in early May and about 9% over the past year. Meanwhile, even with Friday's gains, the stock trades only a touch above tangible book value.

That is because amid the good news, there are reasons for doubt. Some of the items that helped aid the quarter's profit were squishy. The bank also restated first-quarter results after finding a material weakness in internal controls because CIO traders apparently mismarked trades, a stark admission of another worrying failure.

All this underlines how tough it is for investors to get a handle on what is really happening inside big banks and the risks embedded in them. Consider special items that helped offset the trading loss. If you exclude and tax adjust some of those - such as gains related to fluctuations in the bank's own debt and securities disposals - JP Morgan's net would be closer to $3.5bn.

Then there is the fact JP Morgan had just $300m in litigation expense in the quarter, compared with $1.9bn a year earlier. Also, the restatement pushed $460m of the overall trading loss from the second to the first quarter.

And JP Morgan released $2.1bn in loan-loss reserves. The bank has been reducing its reserve in recent quarters as delinquencies and charge-offs decline. But it is virtually impossible for an investor to judge the size and timing of such releases. Not to mention that Mr. Dimon has referred to these releases as "paper profits." Depending on an investor's view of those items, JP Morgan's profit can be whittled down to $1bn or $2bn.

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That is not terrible considering the trading loss and the macro headwinds facing banks. Yet the cookie-jar nature of some of the items is unsettling, especially when combined with the admission of a material weakness in internal controls.

If CIO traders were able to possibly mismark positions, investors can only hope controls are more robust elsewhere. JP Morgan, after all, had $110bn at the end of the first quarter in assets that are marked using internal models, not observable prices.

On top of that, there are growing fears over the burgeoning Libor scandal. JP Morgan had little to say on that, other than that it is cooperating with investigations. Meanwhile, Europe remains an ever-present threat.

So while JP Morgan has weathered the CIO storm and is enjoying some improvement in its core lending businesses, investors in big banks can't yet see the sun. Until they do, it is tough to see even JP Morgan trading at a sizable premium to tangible book.